It could have been a gear-tester horror story—an open-air bivy, between snowdrifts, on a 0°F night in New Hampshire’s White Mountains. But this Arctic-worthy 800-fill down mummy kept our tester off the evening news. “I was warm and comfortable all night, even though the clothes I slept in were sweat-soaked from the day’s hike,” our guinea pig reports. “I usually have trouble keeping my feet warm in winter, but the microfleece-lined footbox eliminated that problem.”

When he awoke, the bag’s exterior was caked in ice, but it dried after 20 minutes in the sun. The extra warmth and superior weatherproofing, thanks to a Drizone waterproof/breathable shell, make this -20°F bag worth the money and weight for hikers who camp in deep-winter conditions: It’s like an insurance policy against bad weather. The cut is efficiently trim, reports our thin, 165-pound tester, but not confining, and the weight-shaving, jacket-style hood fits closely—big guys and restless sleepers should try it in the store. The BTU packs down small compared to similarly warm winter bags (9”x19”) in the included compression sack, and for the weight, few bags deliver more warmth. $529; 4 lbs. 2 oz.; -20°F; sierradesigns.com